Even though there have been significant advances made in recent years in the technology of implementing electromechanical micromirror devices as spatial light modulators, there are still limitations and difficulties when these are employed to provide high quality image displays. Specifically, when the display images are digitally controlled, the image qualities are adversely affected due to the fact that the image is not displayed with a sufficient number of gray scales.
Electromechanical micromirror devices have drawn considerable interest because of their application as spatial light modulators (SLMs). A spatial light modulator requires an array of a relatively large number of micromirror devices. In general, the number of devices required ranges from 60,000 to several million for each SLM. FIG. 1A refers to a digital video system 1, disclosed in a relevant U.S. Pat. No. 5,214,420, that includes a display screen 2. A light source 10 is used to generate light energy for the ultimate illumination of display screen 2. Light 9 generated is further concentrated and directed toward lens 12 by mirror 11. Lens 12, 13 and 14 form a beam columnator, which operates to columnate light 9 into a column of light 8. A spatial light modulator 15 is controlled by a computer 19 through data transmitted over data cable 18 to selectively redirect a portion of the light from path 7 toward lens 5 to display on screen 2. As shown in FIG. 1B, the SLM 15 has a surface 16 that includes an array of switchable reflective elements, e.g., micromirror devices 32, such as elements 17, 27, 37, and 47 as reflective elements attached to a hinge 30. When element 17 is in one position, a portion of the light from path 7 is redirected along path 6 to lens 5, where it is enlarged or spread along path 4 to impinge onto the display screen 2 so as to form an illuminated pixel 3. When element 17 is in another position, light is not redirected towards display screen 2 and hence pixel 3 remains dark.
The on-and-off states of the micromirror control scheme, as that implemented in the U.S. Pat. No. 5,214,420 and by most of the conventional display systems, impose a limitation on the quality of the display. Specifically, in a conventional configuration of the control circuit, the gray scale (PWM between ON and OFF states) is limited by the LSB (least significant bit, or the least pulse width). Due to the on-off states implemented in the conventional systems, there is no way to provide a shorter pulse width than LSB. The least brightness, which determines gray scale, is the light reflected during the least pulse width. The limited gray scales lead to degradations of image display.
Specifically, FIG. 1C exemplifies a conventional circuit diagram of control circuit for a micromirror, according to the U.S. Pat. No. 5,285,407. The control circuit includes memory cell 32. Various transistors are referred to as “M*” where “*” designates a transistor number and each transistor is an insulated gate field effect transistor. Transistors M5, and M7 are p-channel transistors; transistors M6, M8, and M9 are n-channel transistors. The capacitances, C1 and C2, represent the capacitive loads of the memory cell 32. Memory cell 32 includes an access switch transistor M9 and a latch 32a, which is the basis of the static random access switch memory (SRAM) design. All access transistors M9 in a row receive a DATA signal from a different bit-line 31a. The particular memory cell 32 to be written is accessed by turning on the appropriate row select transistor M9, using the ROW signal functioning as a word-line. Latch 32a is formed from two cross-coupled inverters, M5/M6 and M7/M8, which permit two stable states. State 1 is Node A high and Node B low and state 2 is Node A low and Node B high.
The mirror is driven by a drive electrode which abuts a landing electrode formed separately from the drive electrode and is held at a predetermined inclination angle. An elastic “landing chip” is formed on the portion of the landing electrode that makes contact with the mirror and assists in deflecting the mirror in the opposite direction when the control is switched. The landing chip is designed to have the same potential as the landing electrode so that there will be no short circuit through contact. Each mirror formed on a device substrate has a square or rectangular shape, and each side has a length of 4 to 15 um. In this configuration, a portion of the reflected light is reflected not from the mirror surface but from the gaps between the mirrors or other surfaces of the mirror device. These “unintentional” reflections are not applied to project an image, however, are inadvertently generated and may interfere with the reflected light for image display. The contrast of the displayed image is degraded due to the interference generated from these unintentional reflections generated by the gaps between the mirrors. In order to overcome such problems, the mirrors are arranged on a semiconductor wafer substrate with a layout to minimize the gaps between the mirrors. One mirror device is generally designed to include an appropriate number of mirror elements, wherein each mirror element is manufactured as a deflectable mirror on the substrate for displaying a pixel of an image. The appropriate number of elements for displaying an image is configured in compliance with the display resolution standard according to the VESA Standard defined by Video Electronics Standards Association or by television broadcast standards. When a mirror device is configured with the number of mirror elements in compliance with WXGA (resolution: 1280 by 768) defined by VESA, the pitch between the mirrors of the mirror device is 10 μm, and the diagonal length of the mirror array is about 0.6 inches.
The dual-state switching, as illustrated by the control circuit, controls the micromirrors to position either at an ON or an OFF orientation, as that shown in FIG. 1A. The brightness, i.e., the gray scales of display for a digitally control image system, is determined by the length of time the micromirror stays at an ON position. The length of time a micromirror is controlled at an ON position is in turned controlled by a multiple bit word. For simplicity of illustration, FIG. 1D shows the “binary time intervals” when controlled by a four-bit word. As shown in FIG. 1D, the time durations have relative values of 1, 2, 4, 8 that in turn define the relative brightness for each of the four bits, where 1 is for the least significant bit and 8 is for the most significant bit. According to the control mechanism as shown, the minimum controllable differences between gray scales is a brightness represented by a “least significant bit” that maintains the micromirror at an ON position.
For example, assuming n bits of gray scales, one time frame is divided into 2n−1 equal time periods. For a 16.7-millisecond frame period and n-bit intensity values, the time period is 16.7/(2n−1) milliseconds. Having established these times for each pixel of each frame, pixel intensities are quantified such that black is a 0 time period, the intensity level represented by the LSB is 1 time period, and the maximum brightness is 2n−1 time periods. Each pixel's quantified intensity determines its ON-time during a time frame. Thus, during a time frame, each pixel with a quantified value of more than 0 is ON for the number of time periods that correspond to its intensity. The viewer's eye integrates the pixel brightness so that the image appears the same as if it were generated with analog levels of light.
For controlling deflectable mirror devices, the PWM applies data to be formatted into “bit-planes”, with each bit-plane corresponding to a bit weight of the intensity of light. Thus, if the brightness of each pixel is represented by an n-bit value, each frame of data has the n-bit-planes. Then, each bit-plane has a 0 or 1 value for each mirror element. According to the PWM control scheme described in the preceding paragraphs, each bit-plane is independently loaded and the mirror elements are controlled according to bit-plane values corresponding to the value of each bit during one frame. Specifically, the bit-plane according to the LSB of each pixel is displayed for 1 time period.
When adjacent image pixels are shown with a great degree of difference in the gray scales, due to a very coarse scale of controllable gray scale, artifacts are shown between these adjacent image pixels. That leads to image degradations. The image degradations are especially pronounced in the bright areas of display, where there are “bigger gaps” between gray scales of adjacent image pixels. The artifacts are generated by technical limitations in that the digitally controlled display does not provide sufficient gray scales. At the bright areas of the display, the adjacent pixels are displayed with visible gaps of light intensities.
As the mirrors are controlled to be either ON or OFF, the intensity of light of a displayed image is determined by the length of time each mirror is in the ON position. In order to increase the number of gray scales of a display, the switching speed of the ON and OFF positions for the mirror must be increased. Therefore the digital control signals need be increased to a higher number of bits. However, when the switching speed of the mirror deflection is increased, a stronger hinge for supporting the mirror is necessary to sustain the required number of switches between the ON and OFF positions for the mirror deflection. In order to drive the mirrors with a strengthened hinge, a higher voltage is required. The higher voltage may exceed twenty volts and may even be as high as thirty volts. The mirrors produced by applying the CMOS technologies are probably not appropriate for operating the mirror at such a high range of voltages, and therefore DMOS mirror devices may be required. In order to achieve a higher degree of gray scale control, more complicated production processes and larger device areas are required to produce the DMOS mirror. Conventional mirror controls are therefore faced with a technical problem in that accuracy of gray scales and range of the operable voltage have to be sacrificed for the benefits of a smaller image display apparatus.
There are many patents related to light intensity control. These Patents include U.S. Pat. Nos. 5,589,852, 6,232,963, 6,592,227, 6,648,476, and 6,819,064. There are further patents and patent applications related to different light sources. These Patents includes U.S. Pat. Nos. 5,442,414, 6,036,318 and Application 20030147052. U.S. Pat. No. 6,746,123 discloses special polarized light sources for preventing light loss. However, these patents and patent application do not provide an effective solution to overcome the limitations caused by insufficient gray scales in the digitally controlled image display systems.
Furthermore, there are many patents related to spatial light modulation including U.S. Pat. Nos. 2,025,143, 2,682,010, 2,681,423, 4,087,810, 4,292,732, 4,405,209, 4,454,541, 4,592,628, 4,767,192, 4,842,396, 4,907,862, 5,214,420, 5,287,096, 5,506,597, and 5,489,952. However, these inventions have not addressed and provided direct resolutions for a person of ordinary skill in the art to overcome the above-discussed limitations and difficulties.
In view of the above problems, the inventors have disclosed, in US Patent Application 2005/0190429, another method for controlling the deflection angle of the mirror to express gray scales of an image. In this disclosure, the intensity of light obtained during the oscillation period of the mirror is about 25% to 37% of the intensity of light obtained while the mirror is held in the ON position continuously.
According to this control process, it is not necessary to drive the mirror at a high speed. Also, it is possible to provide a higher number of the gray scale using a hinge with a low elastic constant. Hence, such a control makes it possible to reduce the voltage applied to the landing electrode.
A projection apparatus using the mirror device described above is broadly categorized into two types: a single-plate projection apparatus that uses only one spatial light modulator to change the frequency (i.e. wavelength or color) of projected light over time for color display, and a multi-plate projection apparatus that uses a plurality of spatial light modulators to modulate illumination light having different frequencies and combine the modulated light for a color display.
FIG. 1E shows the configuration of a representative single-plate projection apparatus. An illumination optical system 100 includes a light source 120 that produces illumination light 110, a collector lens 130 that converges the illumination light 110, a rod integrator 140, and a second collector lens 150 that focuses the exit plane of the rod integrator onto the device.
The light source 120, the collector lens 130, the rod integrator 140 and the collector lens 150 are disposed, in this order, along the optical axis of the illumination light 110 that is emitted from the light source 120 and incident to the side of a TIR prism 160. The mirror device 210 and the TIR prism 160 are disposed along the optical axis of a projection optical system 200. The illumination light that passes through the light source optical system 100 and enters the TIR prism is reflected off the total reflection surface of the TIR prism 160 and incident on the mirror device 210 at a predetermined inclination angle. Reflected light 220 reflected off the mirror device 210 at a right angle is enlarged and projected on a screen through a projection optical system 230.
There is further provided a wavelength selection filter member 300 that alternately inserts and retracts optical filters that transmit light having different frequencies in the light path of the illumination optical system or the projection optical system. The spatial light modulator is configured to modulate the illumination light based on different color data in synchronization with the insertion and retraction of the wavelength selection filters into and out of the light path. Alternatively, as shown in FIG. 1F, several kinds of light sources 400 emitting lights of different colors, may be implemented and turned on in a time-sequential manner.
The single-plate projection apparatus described above has the advantage of a relatively simple configuration, which allows for easy adjustment. However, it also has the problem of low light usage efficiency because only light of a specific wavelength is used at any one time. Another problem is that since different colors are displayed in a time-sequential manner, if the speed of switching the colors is not fast enough, a color breakup phenomenon occurs in which a viewer perceives each of the colors as a color band.
FIG. 1G shows an exemplary multiple-plate optical configuration. In FIG. 1G, the illumination light from a light source 5210 is incident on the total reflection surface of a TIR (Total internal reflection) prism 5311 at a specified angle and guided to a prism for color composition/separation. The TIR prism 5311 separates the light path of the illumination light from the light path of the modulated light. The color composition/separation prism includes a first color separation/composition prism 5312 and a first joined prism, in which a second color composition prism 5313 is joined with a third color composition prism. The first color separation/composition prism 5312 has a first dichroic film on the exit plane that reflects only red light and transmits other colors. The red illumination light reflected off the first dichroic film is totally reflected off the incidence plane of the color separation/composition prism 5312 and incident to the first spatial light modulator 5100 at a desired angle of incidence. The modulated light that is reflected off the first spatial light modulator 5100 is totally reflected off the incidence plane of the first color separation/composition prism 5312, reflected off the first dichroic film, and enters the projection light path.
Blue and green illumination lights that have passed through the first dichroic film enter the second color separation/composition prism 5313. A second dichroic film that reflects only blue is disposed on the joined surface of the second color separation/composition prism 5313 and the third color separation/composition prism. Therefore, the blue illumination light is separated from the illumination light incident on the second color separation/composition prism 5313 and reflected off the second dichroic film. The reflected blue illumination light is totally reflected off the light incidence plane of the second color separation/composition prism 5313 and incident to the second spatial light modulator 5101. The light modulated by the second spatial light modulator 5101 is reflected off the incidence plane and the second dichroic film and directed to the projection light path. The green light that has passed through the second dichroic film is modulated at a third spatial light modulator 5102 and reflected into the projection light path. The red, blue, and green lights modulated at the first, second and third spatial light modulators 5100, 5101 and 5102 and reflected into the same projection light path pass through the total reflection surface of the TIR prism 5311 and are projected onto a projection surface through a projection lens 5400.
In such a configuration, in which each of the primary colors is projected at all times, as compared to a single-plate projection apparatus described above, there will be no visual problem such as color breakup. Furthermore, effective use of light from the light source provides a bright image. On the other hand, adjustment of the configuration, for example, aligning the spatial light modulators to correspond to the respective color light beams, will be more complex and result in the increased size of the apparatus.
It is therefore desirable to provide a projection apparatus that will not suffer from color breakup using a simple single-plate optical configuration. A method for eliminating the above problem by coloring each micromirror element with a coloring resist was disclosed in U.S. Pat. Nos. 5,168,406, 5,240,818 and 5,452,138.
However, the patented inventions in these patents have further difficulties. Specifically, when the thickness of the coloring resist exceeds a certain thickness, such as when the thickness is one to three micrometers thicker than that of the actual mirror, and furthermore, when a protective layer is provided on the coloring resist, it is difficult to ensure the flatness of the mirror. In this case, as the thickness and hence the mass of the mirror increases, the natural frequency of the mirror increases accordingly, thus resulting in further difficulty in driving the mirror at high speeds.
On the other hand, JP-A-9-101468 discloses a configuration in which a diffraction grating is formed on the mirror. In this case, however, it is necessary to direct diffracted color light beams in the same direction and guide them into a projection lens. The apparatus has a disadvantage due to the complex configuration that leads to higher production costs and further difficulties in control and operation.
The invention has been made in view of the above problems and aims to provide an image projection apparatus having a simple optical configuration that does not suffer from color breakup. The display image projection system of this invention achieves a higher level of display gray scales by providing a sub-wavelength grating (SWG) having a period smaller than the wavelength of light on a mirror so as to form a mirror element that reflects light having a specific wavelength.